Lecture 4. Atmospheric Absorption of Radiation
• The atmosphere absorbs little (5%) of radiation in
visible wavelengths
• Stratospheric ozone absorbs almost all UV radiation
• Water vapor, CO2, methane, nitrous oxide, ozone,
and CFCs all absorb some of the infrared radiation
emitted by the earth, and are called greenhouse
gases.
EOM 2.9
The Greenhouse Effect
H20
CO2
EOM 2.10
• Greenhouse gases absorb and emit infrared (IR, also called longwave) radiation. Some of this IR radiates downward, warming the surface.
• No greenhouse effect ⇒Earth’s surface would average a frigid -18°C (0°F).
• Water vapor, clouds, and CO2 (in that order) produce the most greenhouse
warming, raising Earth’s mean surface temperature to 15°C (59°F).
Clouds’ Greenhouse Effect
• Cloud reflects visible light but strongly absorbs IR radiation, so also has
greenhouse effect.
Low, warm cloud radiates lots of IR groundward (and
upward)⇒ large surface greenhouse effect
Low warm cloud
High, cold cloud radiates less IR groundward (and
upward)⇒ less surface greenhouse effect
• Thus surface temperature tends to cool much less on a
Thin high cloud
night with low or thick clouds.
• Note: Real greenhouses don’t work like the ‘greenhouse
effect’! They trap absorbed heat by suppressing air
exchange with the outside.
• IR satellite images show upwelling IR. White ⇒ low IR, i.
e. high, cold cloud. Grey ⇒ low, warmer, cloud. Black ⇒ clear, can see surface (which is usually warmer than clouds).